The present invention relates to a power factor correction (PFC) controller and an operation method thereof, and more particularly to a controller for a discontinuous mode PFC controller having a power saving modulator and operation method thereof
Most power factor correction techniques incorporate and utilize a boost topology, which can be operated in either a continuous or a discontinuous inductor current mode at a fixed or a variable switching frequency. The continuous inductor current mode operated at a fixed switching frequency is used for higher power applications because of a low peak current applied and operated. For applications below 250 watts, the usage of the discontinuous inductor current mode operated with the variable switching frequency provides several advantages including small inductors, low costs, simple circuits, and zero current switching (ZCS). A pulse width of a power factor correction (PFC) controller is controlled by a voltage error amplifier, which is compared to a saw-tooth waveform generated by the controller. The pulse width varies with line and load conditions, but should be maintained constant for a half of a line cycle. Therefore, it is necessary for the voltage error amplifier to have a lower frequency bandwidth that below the line frequency.
The ZCS includes several advantages in applications. One advantage is that the inductor current must be released to zero before a next switching cycle is started which produces high switching efficiency. Since the change of the inductor current is equal to the peak inductor current and the current starts and returns to zero in each cycle, the current waveform has a triangular shape with an average value equal to one-half of the peak current multiplied by its time. Thus, the peak current is limited to exactly twice the average current. Since ZCS is switched right on the edging of the continuous and discontinuous current modes, the operation will be in variable switching frequency. Determined by the line input and output load, the pulse width of the switching signal is modulated, the switching frequency turns into low for heavy load conditions and becomes high in response to the light load conditions. The low-bandwidth pulse width modulation (PWM) incorporates ZCS to provide a natural power factor correction for the input current.
In recent prior arts, the ZCS varieties of the discontinuous current PFC controllers have been developed for the power factor correction control. Among them, the PFC controllers include the ST6561 of ST-Microelectronics, France; the MC34262 of ON-Semiconductor, Colorado; and the TDA4862 of Siemens, Germany. All of these controllers are designed to operate in a high frequency for a light load condition and/or no load condition. The switching losses of the power converter and the PFC booster are proportional to the switching frequency, in which the switching loss of the switching transistor, the power consumption of the snubber and the inductor losses are increased in accordance with a higher switching frequency. The drawback of the forgoing controllers is that under high frequency operation in the light load condition, it is difficult for the power converter to meet the energy conservation requirement, especially for light load and no load conditions. Therefore, it is desirable to provide a PFC controller that maintains the PFC function and provides low power consumption for light load.
An objective of the invention is to provide a zero current switching (ZCS) discontinuous mode PFC controller to provide high efficiency PFC, as well as reducing the power consumption of PFC controller under light load conditions.
Another objective of the invention is to eliminate the need of start-up resistors, which in turn, saves power.
Another objective of the invention is to provide a method of limiting the maximum output power of the PFC controller for under-voltage protection.
The present invention is related to a ZCS discontinuous mode PFC controller. When the line voltage is applied to the PFC converter, the feedback resistor and a parasitic diode of the controller startup the controller. Once the controller is turned on, a transistor will switch the feedback resistor to turn into the divider of the voltage feedback loop. For the PFC, an external resistor is used to determine the maximum on-time of the switching signal, thus limit the maximum output power. While the switching signal is off; the inductor current will be released to zero before the next switching cycle is started which achieves ZCS. In order to decrease the switching frequency for light load conditions, an off-time delay is inserted right before the start of every switching cycle. The feedback voltage, which is derived from the voltage feedback loop, and the supply voltage are taken as the variable. The off-time delay is modulated to be the function of the feedback voltage and supply voltage. A threshold voltage is a constant that defines the level of the light load. A limit voltage defines the low-level of the supply voltage. Once the feedback voltage decreases and is lower than the threshold voltage, the off-time delay will be increased accordingly. When the supply voltage is lower than the limit voltage, the off-time delay is decreased to inhibit the decrease of the switching frequency, therefore, preventing a low supply voltage. The switching frequency is decreased in accordance with the decrease of the load. Consequently, this reduces the switching losses and power consumption for light load and no load conditions.